Coral Bleaching on the Great Barrier Reef

Has coral bleaching affected the Great Barrier Reef?

A reef bleached white during the 1998 mass bleaching event

The Great Barrier Reef has experienced mass coral bleaching events in the past. During the 1998 global mass bleaching event sea surface temperatures in the Great Barrier Reef reached levels that were the highest ever recorded. Aerial surveys showed that, on average, about 50 per cent of reefs suffered bleaching; 87 per cent of inshore reefs and 28 per cent of mid-shelf and offshore reefs. Inshore reefs suffered the highest mortality rates while mid-shelf and offshore reef corals generally escaped with minimal mortality. Overall about, five per cent of reefs were severely damaged by this bleaching event.

The Great Barrier Reef suffered another mass bleaching event in 2002; this was the largest on record. Two periods of hot weather resulted in sea surface temperatures a few degrees centigrade higher than long-term summer maxima. Aerial surveys revealed that on average 60 per cent of reefs were bleached. Inshore reefs were again the most affected�69 per cent. A greater proportion of mid-shelf and offshore reefs bleached in 2002 than in 1998�51 per cent. Mortality was variable and generally low, but again, about five per cent of reefs were severely damaged.

Bleached landscape of staghorn and plate corals in 2006

More recently, bleaching affected the southern section of the Great Barrier Reef in the summer of 2006. The inshore reefs off Rockhampton were badly affected by the unusually warm water that persisted in the area for over two months. Conditions was especially severe in the Keppel Islands, where over 80 per cent of corals bleached and 40 per cent died as a result of bleaching stress. Fortunately, the remainder of the Great Barrier Reef suffered very little bleaching in that year.

Stressful increases in temperature are predicted to occur more frequently in the future due to climate change. Coral bleaching is therefore one of the most serious long-term threats to the Great Barrier Reef. Current model predictions are for rises in average water temperature in the Great Barrier Reef of between 1 and 3°C over the course of the century. Bleaching events are likely to increase in frequency and severity under these scenarios. Some scientists even predict that mass bleaching could become an annual event.

The Great Barrier Reef Marine Park Authority (GBRMPA) has developed a Coral Bleaching Response Plan to provide a comprehensive strategy for detecting and responding to widespread coral bleaching during summer.

The challenge now is to determine what effects these events will have on reef ecosystems and on the communities and industries that rely on them. The GBRMPA is currently working with leading experts to identify the vulnerability of reef ecosystems to climate change.

The GBRMPA is also working to maintain and restore reef resilience. Initiatives such as the Great Barrier Reef Marine Park Zoning Plan and the Reef Water Quality Protection Plan help to maintain the health and resilience of the Reef. A healthy, resilient reef is better able to resist or adapt to climate change.

For more information on coral bleaching and resilience-based management of coral reefs see A Reef Manager�s Guide to Coral Bleaching.